Revision of Loosened Iliac Screws: A Biomechanical Study of Longer and Bigger Screws

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Abstract

Study Design.

The present study compared the biomechanics of 2 revision iliac screws: longer and bigger screws, on human cadaveric pelves.

Objective.

To determine if a bigger screw resists loosening under cyclic loading better than a longer screw in revising loosened iliac screws.

Summary of Background Data.

Iliac screws have been used in treating spinal deformity, spondylolisthesis and many other spinal diseases. Because of the cancellous bone along the screw trajectory, screw loosening over cyclic loading has been experienced in clinical applications. Two popular revision choices are: a longer screw and a bigger screw. However, their biomechanics has not been characterized. The objective of this study is to determine the rate of loosening of longer or larger revision iliac screws under cyclic loading.

Methods.

Eight pairs of human cadaver pelves were harvested. Each side was randomly assigned for a longer revision screw or a larger revision screw. Because of different bone quality in each specimen, applied load was varied according to the peak insertion torque of the primary iliac screws. The load was applied at an anatomic angle with a frequency of 2 Hz. The motion of screw with respect to the pelvis at the bone entry point was recorded with a motion tracking system. The amount of loosening after a specific number of cycles was determined from the screw motion data.

Results.

The average maximal insertion torque of bigger revision screws (3.2 Nm) was greater than that of longer revision screws (2.7 Nm) with P = 0.03. The average loosening rate was 0.28 ± 0.13 (SE) mm/thousand cycles for longer revision screws and 0.06 ± 0.05 (SE) mm/thousand cycles for bigger revision screws. The difference between these 2 revision screws was significant (P = 0.03). In addition, the bigger revision screws had a lower loosening rate than that of the primary screws (P = 0.03).

Conclusion.

Iliac screws are susceptible to loosening under cyclic loading due to the cancellous bone structure surrounding the screw body. Experimental data showed that the bigger revision iliac screw resists loosening better than the longer screw and the primary screw. Thus, the bigger revision screw is favored if the patient’s anatomy allows such operation.

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